In many situations it is useful to estimate the distance between two wireless stations. For example, if one of the wireless stations is a mobile user terminal which can communicate with several fixed base stations then it may be useful to be able to estimate the distance of the user terminal from each of the fixed base stations and thereby determine its location. In another situation, one of the wireless terminals may be located at a vending machine that can perform financial transactions via a user terminal. In that situation it may be desirable for the vending machine to estimate the distance between it and the user's terminal and to accept transactions only from a user who is within some short distance, perhaps 3 m.
It is not feasible simply to use received signal strength to estimate the distance between the stations, since the output power of the transmitter, the antenna gains, the propagation conditions and the receiver sensitivity can all vary so much that distances derived from received signal strength indications (RSSI) can be out by a factor of 10.
Another method for determining the distance is to measure the time taken for signals to travel between the two stations. This requires highly accurate synchronisation between the stations and highly accurate timing systems, especially if relatively short distances such as a few meters are to be measured.